Being well versed in the correct use of punctuation is a vital tool in the trade of being a professional writer, whether you are in the self publishing field or otherwise. Using punctuation incorrectly - such as adding multiple exclamation or question marks to the end of a sentence when just the one will suffice - is a very bad habit. Why, I hear you ask?!!?? Because it is the clear indication of an amateur and one thing you do not want as a writer is to have anyone - be they other writers, editors, publishers or readers for that matter - to regard you as an amateur. Sometimes it is tempting to add an extra exclamation mark as you may feel that a particular sentence is in need of extra emphasis than one exclamation mark on its own seems to lack somehow, but not only is this very unlikely, but the fact is there are better ways to go about doing this. For instance, bold face, initial capitals, or even italics can be used to add extra emphasis to a sentence without the need for unnecessary additional punctuation. Do You See What I Did There?
Of course, using even one exclamation mark may not be necessary depending on what you are writing in the first place. If you are writing some kind of non-fiction "how to" business guide, it is highly unlikely that you are going to need such marks at all.
p.s. Using punctuation wrongly makes you look like a hack!?;:'",.*